System and method for controlled directional drilling

ABSTRACT

A system and method for controlled directional drilling utilizes a system approach to design the hardware for drilling according to the well plan. The bend angle of a bent housing, connected between the bit and downhole motor, the diameter of a plurality of stabilizers and placement of the stabilizers with respect to the drill bit are selected and predetermined on the basis of the desired well plan. With the use of an MWD, the direction of the progressing borehole is tracked from the surface. Direction changes as required are controlled from the surface simply by controlling rotation of the drillstring. For curved path drilling, only the downhole motor is rotated, causing the borehole to travel along the curve determined by the bend angle in the bent housing and the diameter and location of the concentric stabilizers. When straight hole drilling is required, both the downhole motor and the entire drill string are rotated, effectively nullifying the effect of the bend angle in the bent housing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to improvements in controlleddirectional drilling systems and more particularly pertains to a new andimproved system and method for controlling the directional drilling ofthe borehole in a manner which will allow the borehole to be drilled inconformance with the proposed well plan.

2. Description of the Prior Art

In the field of controlled directional drilling of boreholes, until veryrecently, it had been the practice to use two separate down-holesystems. One system was used for drilling straight holes. A completelydifferent system was used for causing the borehole to turn direction.The use of these two systems required that the entire drillstring betripped or pulled from the borehole so that the down-hole system couldbe changed each time a change of direction was required. This type ofsystem is described in SPE Paper No. 9649 entitled "Kicking Off in LargeDiameter Holes" presented at the 1981 SPE Middle East Oil TechnicalConference held in Manama, Bahrain, Mar. 9-12.

Although such systems produced results, they were unsatisfactory inseveral major respects. Considerable time was wasted as a result of thenon-drilling activity occasioned by having to trip the bottom-holeassembly, either to follow the curvature in the well plan or to makecorrections for unforeseen deviations in the borehole. This type ofoperation considerably increased the drilling time and decreased therate of penetration (ROP). Another problem with this system is that thestandard straight-hole drilling, bottom-hole assemblies utilized,deviated, sometimes considerably, from the well plan. In such a case,not only will the driller not reach his target but he will end up with acrooked hole, worn casing, stuck pipe, and expensive fishing jobs.

Deviating boreholes have been a subject of concern to this industry fora long time. Many approaches have been tried to first understand themultifacet problem and then to come up with a workable solution. Oneexample of such an approach can be found in SPE Article No. 5070entitled "Factors Affecting the Control of Borehole Angle in Straightand Directional Wells" presented at the SPE-AIME 49th Annual FallMeeting in Houston, Tex., Oct. 6-9, 1974.

One of the problems confronting the industry with respect to thedrilling of directional boreholes and deviation of boreholes from a wellplan was obtaining arcurate information about the direction of theborehole. As a result, the industry developed systems for monitoringwhile drilling (MWD systems). In order to be able to ascertain when aborehole is deviating from its well plan and to be able to ascertain andcontrol the direction of the borehole in order to follow the well plan,many different types of monitoring systems were developed.

A certain number of these systems are discussed, for example, in SPEpaper No. 9224, entitled "A Case Study Comparison of Wells Drilled Withand Without MWD Directional Surveys on the Claymore Platform in theNorth Sea" first presented at the SPE 55th Annual Technical Conferenceand Exhibition in Dallas on Sept. 21-24, 1980. Another example can befound in SPE paper No. 10053 entitled "Mud Pulse MWD Systems Report"first presented at the SPE 56th Oct. 4-7, 1981.

Another aspect of the problem confronting anyone attempting to drill adirectional well at increased ROP without increasing cost, was therequirement of a powerful down-hole motor to turn the borehole, whenrequired by the well plan, or to bring a deviated borehole back to thewell plan. Such motors have been the focus of industry attention forconsiderable time and are now, to a reasonable extent, available. Adescription of a type of down-hole motor available at the present timecan be found in SPE paper No. 13026 entitled "PDM vs. Turbodrill: ADrilling Comparison", presented at the 59th Annual Technical Conferenceand Exhibition in Houston, Tex. Sept. 16-19, 1984.

Even with all of these available pieces of a bottom hole assembly, theprocedure for drilling a directional well was still to trip the drillstring when a change in the direction of the borehole was called for bythe well plan. In spite of the ability to monitor while drilling thedirection of the borehole, the use of powerful downhole motors, and theability to modify various factors like bit weight while the bottom-holeassembly was in the hole, boreholes still deviated from the well plan,requiring tripping of the drillstring and adjustment of the bottom-holeassemblies, as well as adjustment of the stabilizers on the drillstring.A method was needed which would considerably reduce, if not eliminateentirely, the round trips required with kick-off techniques and assemblychanges for directional control of the borehole. A technique which showspromise and is currently being utilized by various operators in theindustry is described in a paper by A. W. Kamp and R. Feenstra entitled"A Technique for Continuously Controlled Directional Drilling" presentedat the Drilling Technology Conference of the International Associationof Drilling Contractors in Dallas, Tex. on Mar. 19-21, 1984. Thetechnique involves the use of a powerful down-hole motor and variousways of creating a side force on the bit or tilting the axis of the bitwith respect to the axis of the borehole. It has been found that the bitwill drill straight when both the drill string and the motor arerotated, and the bit will deviate in a desired direction when only themotor is rotated and the drillstring is kept stationary in a controlledtool face direction.

Since the introduction of this technique, various entities in thisindustry have developed a variety of bottom-hole assemblies to takeadvantage of its possibilities. Some systems have proven more promisingthan others. Each system in its own way, is searching for a reduction indrilling time by increasing rate of penetration and thereby reducing thecost of the well. One such system is described in an article entitled"Drill Faster, More Accurately With New Navagation System" published inWorld Oil on Aug. 1, 1985 and authored by T. Brassfield and H. Karlson.

The present invention is an improvement over the systems presentlyavailable and being tried by the industry to increase ROP of adirectional well. The improved performance of the present invention isbased on the fact that an overall system approach to each drilling jobis utilized. In other words, the bottom-hole assembly is uniquelytailored for each proposed well plan by taking into consideration amyriad of facts such as hole condition, pump data, type of mud beingutilized, type of formation being drilled, drilling assembly components,drilling flow rate, well plan, i.e. direction of the borehole afterdeviating from vertical, in addition to information about the drillingbit which includes bit size, bit type, bit pressure drop, and gaugelength, as well as degrees of offset of the center line of the bit facefrom the center line of the borehole. This information is utilizedaccording to the present invention to come up with a bottom holeassembly and method of building a bottom hole assembly which provides anROP for directional wells which is considerably higher than washeretofore possible.

SUMMARY OF THE INVENTION

A system approach to the design of a down-hole assembly for directionaldrilling requires establishing the value for a series of importantvariables on the basis of the proposed well plan. The major variableswhich are systematically determined are bit offset from center,determined by the angle of bend in a bent housing located between themotor and the bit, exact placement along the drillstring of a pluralityof concentric stabilizers with respect to the bit, diametric size ofeach concentric stabilizer with respect to the diameter of the borehole,and to a lesser degree weight on the bit. The entire system, whenassembled for a particular well plan is capable of following that wellplan with only slight directional correction in the borehole.Directional corrections are made and control of the system is maintainedby rotating the down-hole motor only, for curved travel of thedrillstring, and rotating the motor and drill string together forstraight travel of the drillstring.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and many of the attendant advantages of this invention willbe readily appreciated and become readily apparent as the same becomesbetter understood by reference to the following detailed description,when considered in conjunction with the accompanying drawings, and inwhich like reference numerals designate like parts throughout thefigures thereof and wherein:

FIG. 1 is a diagramatic illustration of the basic components of thebottom-hole assembly of the present invention;

FIG. 2 is a diagramatic illustration showing how the bit offset isobtained in the bottom-hole assembly according to the invention;

FIG. 3 is a vector illustration showing how the bottom hole assembly, ofthe present invention drills in a controlled direction;

FIG. 4 is a table and component diagram for a bottom-hole assembly builtaccording to the present invention illustrating the interrelationship ofthe basic components of the bottom-hole assembly;

FIG. 5 is an alternate table and diagram illustrating a differentrelationship between the basic components of the bottom-hole assembly ofthe present invention; and

FIG. 6 is a vertical section of a borehole showing the accuracy withwhich the actual borehole follows the proposed well plan.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the basic components of the bottom holeassembly 11 of the present invention are illustrated. The borehole 13 isshown in an oversized and exaggerated manner and is illustrated as beingcapable of moving in three dimensional space as defined by the Cartesiancoordinates x, y and z. The z axis is for the purposes of illustration,defined as the center line of the borehole 13.

The first element of the bottom-hole assembly of the present inventionis the drill bit 15 which is connected to a shaft that is concentricallylocated within a bearing assembly 17. This shaft is in turn connectedthrough a bent housing 21 to the output shaft of the down-hole motor 25.The housing of the down-hole motor 25 is in turn connected to the drillstring casing 27 which extends all the way to the surface of theborehole 13 and is in turn connected to a means for rotating the entireassembly from the surface (not shown). The bottom-hole assembly,according to the present invention, also includes at least three andpreferably four stabilizers 19, 29, 31 and 33 precisely located alongthe drill string with respect to the drill bit 15 and with respect toeach other.

An important thing to remember with respect to the illustration of FIG.1 is that the centerline of the bit 15 is offset from the centerline ofthe borehole 13 in an amount determined by the offset 35 (FIG. 2) whichis fixed by the bent housing 21, 23 that is connected between thedown-hole motor 25 and the motor bearing assembly 17. The utilization ofa bent housing 21 at this precise point in conjunction with theconcentric stabilizers as shown in FIG. 1 has proven to be a majorfactor in increasing the rate of penetration of this particularbottom-hole assembly beyond that heretofore available.

Referring now to FIG. 2, the bit offset created by the bent housing 21,23 is illustrated in more detail. FIG. 2 illustrates the turningmechanism of the bottom-hole assembly built according to the presentinvention. This turning mechanism includes the bent housing 21, 23having a specific tilt angle 35 and a concentric stabilizer 19 locateddown-hole of the tilt point 23 on the bent housing 21, 23, and veryclose to bit 15 on bearing housing 17. The drive shaft for bit 15 isconcentric within bearing housing 17, resulting in an offset 35 of thecenter of the face of the bit 15 from the centerline of the borehole 13by an angle α which is the tilt angle 35 of the bent housing 21.

The down-hole motor 25 utilized with this type of arrangement ispreferably a positive displacement motor of the type described in theSPE paper No. 13026 entitled "PDM Versus Turbo-Drill: A drillingcomparison". The concentric stabilizer 19 located close to the bit 15serves mainly to maintain the bit offset angle 35 by minimizing thedeflections which might increase or decrease this offset angle.

Although all the elements of the bottom-hole assembly, as shown in FIG.1 of the present invention, affect the direction of the borehole thatwill be drilled by the bottom-hole assembly, it is convenient toconsider the bearing stabilizer 19 and the bent housing 21, 23 as thekey factors in determining the extent to which the bore-hole willdeviate from the vertical. Experience and mathematical modeling have infact born out this analytical simplification.

Referring now to FIG. 3, a curve which is made up of a plurality ofsegments 37, 45, 49, 53 and 57 is illustrated as the curve along whichthe bottom-hole assembly of the present invention will travel asdetermined by the elements of the bottom-hole assembly including bearingstabilizer 19 and the other stabilizers making up the bottom-holeassembly. The bottom most three stabilizers can be thought of asdefining points on a circle which determine the radius of the circle. Aportion of the circumference of this circle is illustrated in FIG. 3 asthe path of travel of the bottom-hole assembly. The vertical distance 59for the curved path traveled is for convenience considered to be asegment of 100 feet.

The initial deviation from vertical 39 of curved segment 37 isdetermined by the bit offset 41 which is controlled by a tilt angle ofbent housing 21, 23. The bit 15 will travel along this offset path 37for a length 61 which is approximately equal to the length of thebearing assembly 17. Whereupon the bit will again follow its offset 41to drill the next straight segment 45 rather than continue straightalong segment 43, and so on to segments 53 and 57. The composite resultis a curved path which deviates from the original vertical 39 by a totalangle in degrees which is related to the angle of offset 41 created bythe tilt angle in bent housing 21, 23.

Referring once more to FIG. 1, the bearing stabilizer 19 and the benthousing 21, 23 is considered the part of the overall system which givesthe bottom-hole assembly the capability of turning left or right in acontrolled manner. The three concentric stabilizers 19, 29 and 31 and,preferably the fourth concentric stabilizer 33 can be considered as thepart of the bottom-hole assembly which gives the assembly the ability tomaintain a straight course, or to build or drop angle. Thus, thebottom-hole assembly of the present invention is really a uniquecombination of two overlapping systems which are integrated to providethe bottom-hole assembly with its unique performance capabilities.

The selection of the diameter and placement of the concentricstabilizers 19, 29, 31 and 33, as well as the tilt angle α of the benthousing 21, 23 are the key factors in determining the performance of thebottom-hole assembly of the present invention.

As a result of mathematical modeling with the aid of a computer andfield experience, it has been possible to come up with a definition ofthe interrelationship between the stabilizers and the offset angle α ofthe bent housing in order to achieve a specific drilling direction. FIG.4 illustrates one such set of relationships.

The basic down-hole assembly components are the drill bit 63, theconcentric stabilizers 65, 69, 71 and 73, and the bent housing 67,having an offset angle α. Performance of this bottom-hole assembly, ischanged by varying the distance of each stabilizer from the bit 63. Thatis the distance L1 of stabilizer 65 from bit 63, the distance L1+L2 ofthe stabilizer 69 from the bit 63, the distance L1+L2+L3 of thestabilizer 71 and the distance L1+L2+L3+L4 of the stabilizer 73 from thebit 63. The angle α is an important contributing factor, as well as theamount of undersize Δd 75 of each stabilizer with respect to the holesize. The amount of weight on bit (WOB) is a factor, as are variousother variables mentioned above, to a minor extent.

Consider now various examples of bottom-hole assemblies which forconvenience are designated as assemblies A, B and C. Considering firstassembly A, the hole size is given as 121/4 inches. Hole washout, as aresult of the bottom-hole assembly will be negligable. This bottom-holeassembly utilizes a bent housing which has a bit offset angle α of 1/2of a degree. The placement of the four stabilizers is as follows. Thebearing stabilizer 65 is located a distance L1 from the bit which isequal to 41/2 feet. Stabilizer 69 is located a distance L2 from thestabilizer 65 which is a distance of 31 feet. Stabilizer 71 is located adistance L3 from stabilizer 69 which is a distance of 45 feet.Stabilizer 73 is located a distance L4 from stabilizers 71 which is adistance of 35 feet. Each of the stabilizers are concentric andundersized with respect to the hole diameter an amount Δd which is equalto 0.032 inches. The location of the four stabilizers 65, 69, 71 and 73at these specific distances with respect to the bit 63 has been found tocreate a system that will build 0.30 degrees per 100 feet regardless ofthe variation of the weight on bit from 10,000 pounds to 40,000 pounds.

Looking now at the bearing concentric stabilizer 65 and the offset angleα, this combination causes the bottom-hole assembly to build angle at0.58 degrees per 100 feet. As a result, the system, when being utilizedin a directional drilling mode, will build angle at 0.88 degrees per 100feet. It can be seen that prior to putting this bottom-hole assemblyinto the ground, its performance in the directional mode can be fairlyaccurately predicted.

Looking now at bottom-hole assembly B, we can see that changing just 2parameters creates a different directional characteristic. Bottom-holeassembly B has an offset angle α of half a degree and utilizes a bearingstabilizer 65 which is undersized by 0.157 inches. All the otherparameters remain the same. The four stabilizers thereby provide abottom-hole assembly which drops angle at 0.59 degrees per 100 feet. Thebent housing and bearing stabilizer causes the bottom-hole assembly tobuild angle by 0.75 degrees per 100 feet. The resulting overall systemwill therefore build angle by 0.16 degree per 100 feet when in thedirectional mode.

Referring to bottom-hole assembly C, again we change the offset angle αand the undersize differential of the bearing stabilizer 65. The angle αis chosen to be 3/4 of a degree and the bearing stabilizer 65 isundersized by 0.282 inches. As a result, the characteristic of thestabilizer string is to drop angle by 1.48 degree per 100 feet, up to1.49 degrees per 100 feet, if the weight on bit is increased from 10,000pounds. The bent housing and bearing stabilizers 65 will build angle at0.92 degrees per 100 feet, up to 0.95 degrees per 100 feet if the weighton bit is increased to 40,000 pounds. Accordingly, the combinationresults in a directional bottom-hole assembly which will drop angle at0.56 degrees per 100 feet.

FIG. 5 illustrates three more bottom-hole assemblies D, E and F. Thebottom-hole assembly D, utilizes an offset angle α of a quarter of adegree and stabilizer spacing of L1--41/4 feet, L2--31 feet, L3--35feet, and L4--45 feet, with an undersized diameter differential Δd of0.032 for each of the concentric stabilizers. This system is shown tobuild angle at 1.01 degrees per 100 feet up to 1.22 degrees per 100 feetas the weight on bit is increased to 40,000 pounds. The bent housing andbearing stabilizer 65 will cause the bottom-hole assembly to build angleby 1.27 degrees per 100 feet up 1.50 degrees per 100 feet as theweight-on bit is increased to 40,000 pounds. As a result, thebottom-hole assembly D will build angle from 2.28 degrees per 100 feetto 2.72 degrees per 100 feet depending upon the amount of weight-on bit.

The bottom assembly E is shown as utilizing an offset angle α of 1/2 adegree and a Δd for bearing stabilizer 65 of 0.157 inches. All othervariables remain the same. The stabilizer section of the bottom-holeassembly, as a result, will build angle at 0.14 degrees per 100 feet upto 0.33 degrees per 100 feet, depending upon the weight on a bit. Thebent housing portion of the bottom-hole assembly will tend to buildangle at 1.44 degrees per 100 feet up to 1.68 degrees per hundred feetdepending on the weight on bit. The overall system will tend to drilldirectionally at 1.58 degrees per 100 feet up to 2.01 degrees per 100feet depending upon the weight on the bit.

Looking now at system F, the bent housing used has an offset angle α of3/4 of a degree and a Δd undersized bearing stabilizer 65 at 0.282inches. All other variables remain the same. As a result, the stabilizersection will drop angle from 0.72 degrees per 100 feet to 0.56 degreesper 100 feet depending on weight on bit. The bent housing will tend tobuild angle at 1.61 degree per 100 feet to 1.87 degree per 100 feet. Thecombination will drill directionally to build angle of 0.89 degrees per100 feet up to 1.31 degrees per 100 feet depending upon weight on bit.

The particular down-hole system chosen, A, B, C, D, E, or F, or anyother system, will depend upon a proposed well plan directed by thecustomer. A typical well plan is shown in FIG. 6 where the borehole isdrilled vertically for approximately 1,850 feet from the surface 77, atwhich point it is kicked off and then drilled at a certain angle to avertical depth of 6,300 feet and an angle depth of 7,970 feet.

The bottom-hole assembly of the present invention is assembled at thesurface with the concentric stabilizers located at distances L1, L2, L3,and L4 and having a differential undersize as specified, and a specificoffset angle α to accomplish the kickoff at 1,850 feet and follow thewell plan as shown in FIG. 6. For straight hole drilling from groundlevel 77 to the 1,850 foot depth, both the down-hole motor and thedrillstring are rotated together. Rotation of the drillstring nullifiesthe directional characteristic built-in to the down-hole assembly. Atthe 1,850 foot mark, where kick-off is required, only the down-holemotor is rotated causing the down-hole assembly to take on its fulldirectional characteristic, kick-off and follow the well plan. Oncecomplete kickoff is established, the drillstring can again be rotated ifthe down-hole assembly starts to build too great an angle. In this waythe down-hole assembly is steered to its target. The result, as thecurves of FIG. 6 illustrate, is that the gyrosurvey data 83 is almostoverlaying the proposed well plan 79.

The actual results of the bottom-hole assembly of the present inventionwere surprising as is evident from this example. The well plan requiredthat the downhole assembly maintain 43 degrees per 56 feet of deviationangle from a depth of 3,077 feet to a depth of 7,216 feet in an 81/2inch hole. The bottom-hole assembly of the present system was used withan offset angle of 1/2 degree. The average rate of penetration of thebottom-hole assembly was 103.5 feet per hour. The rate of penetrationwhile drilling was 147 feet per hour which reached up to 330 feet perhour. The system hit the target 6 feet under average angle and 40 feetto the right. Total cost savings was $112,500 as a result of being 3/4of a day ahead of schedule.

As can be seen from this example, by taking a systems approach to thebottom-hole assembly to be used in directional drilling and specificallydesigning the bottom-hole assembly for a particular well plan in themanner illustrated and according to the present invention, the ROP canbe increased considerably, resulting in significant savings per well.

What is claimed is:
 1. An improved system for controlled directional andstraight drilling of a borehole wherein the entire drillstring isrotatable from the surface, including a bottomhole assembly comprised ofa drill bit connected to the output shaft of a downhole motor forindependent rotation from the drillstring, said motor connected at itsuphole end to the drillstring, the improvement in the bottomholeassembly, comprising:a first, concentric stabilizer, having apreselected diameter slightly smaller than the diameter of the borehole,mounted between the drill bit and motor at a preselected distance fromthe bit around the output shaft of said downhole motor, said shaft beingconcentrically located therein; a .[.heat.]. .Iadd.bent .Iaddend.motorhousing connected to the downhole end of said downhole motor that has abend from its geometric center at a predetermined angle α at itsdownhole end, thereby offsetting the centerline of said first stabilizerand the centerline of the drill bit face from the borehole centerline bysaid angle α; a second concentric stabilizer, having a preselecteddiameter slightly smaller than the diameter of the borehole, mounted ata preselected distance from said first concentric stabilizer on thedrillstring, said drillstring being concentrically located therein; anda third concentric stabilizer, having a preselected diameter slightlysmaller than the diameter of the borehole, mounted at a preselecteddistance from said second concentric stabilizer on the drillstring, saiddrillstring being concentrically located therein; wherein the angle α ofthe bend in the motor housing, and the diameter and placement of theconcentric stabilizers are determined by the desired path of theborehole to be drilled, said system drilling a curved borehole when onlythe downhole motor is activated, and drilling a straight borehole whenthe downhole motor is activated and both the drillstring and downholemotor housing are rotated.
 2. The system for controlled directional andstraight drilling of a borehole of claim 1 further comprising a bearinghousing connected between the drill bit and the downhole end of saiddownhole motor housing, the drive shaft of said downhole motor beingconcentrically located therein and connected to drive said drill bit,said first concentric stabilizer being mounted on said bearing housingat a distance less than five feet from the face of the drill bit.
 3. Thesystem for controlled directional and straight drilling of a borehole ofclaim 2 wherein the bend angle α of the downhole motor housing is onedegree or less.
 4. The system for controlled directional and straightdrilling of a borehole of claim 1 further comprising a fourth concentricstabilizer located a preselected distance on the drillstring from thethird stabilizer.
 5. The system for controlled directional and straightdrilling of a borehole of claim 1, wherein the bend angle α of thedownhole motor housing is about one-quarter of one degree.
 6. The systemfor controlled directional and straight drilling of a borehole of claim1, wherein the bend angle α of the downhole motor housing is aboutone-half of one degree.
 7. The system for controlled directional andstraight drilling of a borehole of claim 1, wherein the bend angle α ofthe downhole motor housing is about three-quarters of one degree.
 8. Thesystem for controlled directional and straight drilling of a borehole ofclaim 5, or claim 6, or claim 7, wherein said first concentricstabilizer is placed at four and one-quarter feet from the face of thedrill bit.
 9. The system for controlled directional and straightdrilling of a borehole of claim 8, wherein said second concentricstabilizer is placed at thirty-one feet from said first concentricstabilizer.
 10. The system for controlled directional and straightdrilling of a borehole of claim 9, wherein said third concentricstabilizer is placed at forty-five feet from said second concentricstabilizer.
 11. The system for controlled directional and straightdrilling of a borehole of claim 9, wherein said third concentricstabilizer is placed thirty-five feet from said second concentricstabilizer.
 12. The system for controlled directional and straightdrilling of a borehole of claim 1, wherein said first concentricstabilizer is placed at four and one-quarter feet from the face of thedrill bit.
 13. The system for controlled directional and straightdrilling of a borehole of claim 12, wherein said second concentricstabilizer is placed at thirty-one feet from said first concentricstabilizer.
 14. The system for controlled directional and straightdrilling of a borehole of claim 13, wherein said third concentricstabilizer is placed at forty-five feet from said second concentricstabilizer.
 15. The system for controlled directional and straightdrilling of a borehole of claim 13, wherein said third concentricstabilizer is placed at thirty-five feet from said second concentricstabilizer.
 16. The system for controlled directional and straightdrilling of a borehole of claim 14 or claim 15, wherein said first,second and third concentric stabilizers are 0.032 inches smaller thanthe borehole.
 17. The system for controlled directional and straightdrilling of a borehole of claim 14 or claim 15, wherein said firstconcentric stabilizer is 0.157 inches smaller than the borehole and saidsecond and third stabilizers are 0.032 inches smaller than the borehole.18. The system for controlled directional and straight drilling of aborehole of claim 14 or claim 15, wherein said first concentricstabilizer is 0.282 inches smaller than the borehole and said second andthird stabilizers are 0.032 inches smaller than the borehole.
 19. Amethod for controlled directional and straight drilling of a borehole,according to a predetermined well plan, utilizing a bottomhole assemblyconnected to a drillstring comprising a drill bit connected forindependent rotation from said drillstring to the output shaft of adownhole motor having its uphole side connected to the drillstring,comprising the steps of:1. selecting the placement and diameter of threeconcentric stabilizers on the drillstring as follows, the firstconcentric stabilizer being placed less than five feet from the face ofthe drill bit and having a slightly smaller diameter than the borehole,the second concentric stabilizer being placed a predetermined distancefrom the first stabilizer and having a slightly smaller diameter thanthe borehole, the third concentric stabilizer being placed apredetermined distance from the second stabilizer and having a slightlysmaller diameter than the borehole;
 2. selecting a predetermined weighton bit;whereby the assembled concentrically stabilized drillstring withthe selected weight on the bit exhibit a certain build angle or dropangle characteristic;
 3. selecting a bend angle under two degrees; 4.placing the bend angle in the drillstring between the downhole motor andthe drill bit on the uphole side of said first concentricstabilizer;whereby said first concentric stabilizer and said selectedbend angle exhibit a certain build angle characteristic, theconcentrically stabilized drillstring with the selected weight on bitand the selected bend angle combining to form an interacting bottomholeassembly having a certain build angle or drop angle characteristicuniquely suited to the well plan; and
 5. steering said unique downholeassembly by: (a) turning the drillstring to point the bend in thedrillstring and the bit in the direction the borehole should follow; (b)activating the downhole motor while keeping the drillstring stationarywhen it is desired to drill along a curved path, as determined by thebend angle and first concentric stabilizing combination; (c) rotatingthe drillstring and downhole motor while activating the downhole motorto turn the drill bit when it is desired to drill along a straight path..Iadd.
 20. An improved steerable system for controlled directional andstraight drilling of a borehole wherein the entire drillstring isrotatable from the surface, including a bottomhole assembly comprised ofa drill bit connected to the output shaft of a downhole motor forindependent rotation from the drillstring, said downhole motor connectedat its uphole end to the drillstring, the improvement in the bottomholeassembly comprising:a concentric stabilizer, having a preselecteddiameter slightly smaller than the diameter of the borehole, mounted ata preselected distance from the drill bit, the output shaft of saiddownhole motor being concentrically located within the concentricstabilizer; and a bent housing connected above said concentricstabilizer and having a bend from its geometric center at apredetermined bend angle α, thereby offsetting the centerline of saidconcentric stabilizer and the centerline of the drill bit face from theborehole centerline by said predetermined bend angle α;wherein thepredetermined bend angle α of the bend in the bent housing, and thediameter and placement of the concentric stabilizer are determined bythe desired path of the borehole to be drilled, said system drilling acurved borehole when only the downhole motor is activated, and drillinga straight borehole when the downhole motor is activated and thedrillstring is rotated. .Iaddend. .Iadd.
 21. A method for controlleddirectional and straight drilling of a borehole, according to apredetermined well plan, utilizing a bottomhole assembly connected to adrillstring comprising a drill bit connected for independent rotationfrom said drillstring to the output shaft of a downhole motor having itsuphole end connected to the drillstring, comprising the steps of:1.selecting the placement and diameter of a concentric stabilizer, theconcentric stabilizer being placed close to the face of the drill bitand having a slightly smaller diameter than the borehole, the outputshaft of said downhole motor being concentrically located within theconcentric stabilizer;
 2. selecting a predetermined weight on the drillbit; whereby the assembled concentrically stabilized bottomhole assemblywith the weight on the drill bit exhibits a certain build angle or dropangle characteristic;
 3. selecting bend angle;
 4. placing the bend angleabove the drill bit on the uphole end of said concentricstabilizer;whereby said concentric stabilizer and said bend angleexhibit a certain build angle characteristic, the concentricallystabilized bottomhole assembly with the weight on the drill bit and thebend angle combining to form an interacting bottomhole assembly having acertain build angle or drop angle characteristic uniquely suited to thewell plan; and
 5. steering said unique bottomhole assembly by: (a)turning the drillstring to point the drill bit in the desired directionthe borehole should follow and activating the downhole motor whilekeeping the drillstring stationary when it is desired to drill along acurved path, as determined by the bend angle and concentric stabilizercombination; (b) rotating the drillstring while activating the downholemotor to turn the drill bit when it is desired to drill along a straightpath. .Iaddend. .Iadd.22. A method for controlled directional andstraight drilling of a borehole, according to a predetermined well plan,utilizing a bottomhole assembly connected to a drillstring comprising adrill bit connected for independent rotation from said drillstring tothe output shaft of a downhole motor having its uphole end connected tothe drillstring, comprising the steps of:
 1. selecting the placement anddiameter of a concentric stabilizer, the concentric stabilizer beingplaced close to the face of the drill bit and having a slightly smallerdiameter than the borehole, the output shaft of said downhole motorbeing concentrically located within the concentric stabilizer; 2.selecting a predetermined weight on the drill bit;whereby the assembledconcentrically stabilized bottomhole assembly with the weight on thedrill bit exhibits a certain build angle or drop angle characteristic;3. selecting bend angle;
 4. placing the bend angle above the drill biton the uphole end of said concentric stabilizer;whereby said concentricstabilizer and said bend angle exhibit a certain build anglecharacteristic, the concentrically stabilized bottomhole assembly withthe weight on the drill bit and the bend angle combining to form aninteracting bottomhole assembly having a certain build angle or dropangle characteristic uniquely suited to the well plan; and
 5. steeringsaid unique bottomhole assembly by activating the downhole motor andselectively rotating the drillstring. .Iaddend. .Iadd.23. An improvedsystem for controlled directional and straight drilling of a boreholewherein the entire drillstring is rotatable from the surface, includinga bottomhole assembly comprised of a drill bit connected to the outputshaft of a downhole motor for independent rotation from the drillstring,said downhole motor connected at its uphole end to the drillstring, theimprovement int he bottomhole assembly comprising:a first concentricstabilizer, having a preselected diameter slightly smaller than thediameter of the borehole, mounted at a preselected distance from thedrill bit, the output shaft of said downhole motor being concentricallylocated within the first concentric stabilizer; a bent housing connectedabove said first concentric stabilizer and having a bend from itsgeometric center at a predetermined bend angle α, thereby offsetting thecenterline of said first concentric stabilizer and the centerline of thedrill bit face from the borehole centerline by said predetermined bendangle α; and a second concentric stabilizer, having a preselecteddiameter slightly smaller than the diameter of the borehole, mounted ata preselected distance above said first concentric stabilizer; whereinthe predetermined bend angle α of the bend in the bent housing, and thediameter and placement of the concentric stabilizers are determined bythe desired path of the borehole to be drilled, said system drilling acurved borehole when only the downhole motor is activated, and drillinga straight borehole when the downhole motor is activated and thedrillstring is rotated. .Iaddend. .Iadd.24. A method for controlleddirectional and straight drilling of a borehole, according to apredetermined well plan, utilizing a bottomhole assembly connected to adrillstring comprising a drill bit connected for independent rotationfrom said drillstring to the output shaft of a downhole motor having itsuphole end connected to the drillstring, comprising the steps of:1.selecting the placement and diameter of two concentric stabilizers asfollows, the first concentric stabilizer being placed close to the faceof the drill bit and having a slightly smaller diameter than theborehole, the output shaft of said downhole motor being concentricallylocated within the first concentric stabilizer, and the secondconcentric stabilizer being placed a predetermined distance above thefirst concentric stabilizer and having a slightly smaller diameter thanthe borehole;
 2. selecting a predetermined weight on the drill bit;whereby the assembled concentrically stabilized bottomhole assembly withthe weight on the drill bit exhibits a certain build angle or drop anglecharacteristic;
 3. selecting bend angle;
 4. placing the bend angle abovethe drill bit on the uphole end of said first concentricstabilizer;whereby said first concentric stabilizer and said bend angleexhibit a certain build angle characteristic, the concentricallystabilized bottomhole assembly with the weight on the drill bit and thebend angle combining to form an interacting bottomhole assembly having acertain build angle or drop angle characteristic uniquely suited to thewell plan; and
 5. steering said unique bottomhole assembly by: (a)turning the drillstring to point the drill bit in the desired directionthe borehole should follow and activating the downhole motor whilekeeping the drillstring stationary when it is desired to drill along acurved path, as determined by the bend angle and first concentricstabilizer combination; (b) rotating the drillstring while activatingthe downhole motor to turn the drill bit when it is desired to drillalong a straight path. .Iaddend. .Iadd.25. A method for controlleddirectional and straight drilling of a borehole, according to apredetermined well plan, utilizing a bottomhole assembly connected to adrillstring comprising a drill bit connected for independent rotationfrom said drillstring to the output shaft of a downhole motor having itsuphole end connected to the drillstring, comprising the steps of: 1.selecting the placement and diameter of two concentric stabilizers asfollow, the first concentric stabilizer being placed close to the faceof the drill bit and having a slightly smaller diameter than theborehole, the output shaft of said downhole motor being concentricallylocated within the first concentric stabilizer, and the secondconcentric stabilizer being placed a predetermined distance above thefirst concentric stabilizer and having a slightly smaller diameter thanthe borehole;
 2. selecting a predetermined weight on the drill bit;whereby the assembled concentrically stabilized bottomhole assembly withthe weight on the drill bit exhibits a certain build angle or drop anglecharacteristic;
 3. selecting bend angle;
 4. placing the bend angle abovethe drill bit on the uphole end of said first concentricstabilizer;whereby said first concentric stabilizer and said bend angleexhibit a certain build angle characteristic, the concentricallystabilized bottomhole assembly with the weight on the drill bit and thebend angle combining to form an interacting bottomhole assembly having acertain build angle or drop angle characteristic uniquely suited to thewell plan; and
 5. steering said unique bottomhole assembly by activatingthe downhole motor and selectively rotating the drillstring. .Iaddend..Iadd.26. An improved steerable system for controlled directional andstraight drilling of a borehole wherein the entire drillstring isrotatable from the surface, including a bottomhole assembly comprised ofa drill bit connected to the output shaft of a downhole motor forindependent rotation from the drillstring, said downhole motor connectedat its uphole end to the drillstring, the improvement in the bottomholeassembly comprising:a concentric stabilizer, having a preselecteddiameter slightly smaller than the diameter of the borehole, mounted ata preselected distance from the drill bit, the output shaft of saiddownhole motor being concentrically located within the concentricstabilizer; and a bent housing connected above said concentricstabilizer and having a bend from its geometric center at apredetermined bend angle α, said bend angle α being oriented insubstantially a single direction from the borehole centerline, therebyoffsetting the centerline of said concentric stabilizer and thecenterline of the drill bit face from the borehole centerline by saidpredetermined bend angle α; wherein the predetermined bend angle α ofthe bend in the bent housing, and the diameter and placement of theconcentric stabilizer are determined by the desired path of the boreholeto be drilled, said system drilling a curved borehole when only thedownhole motor is activated, and drilling a straight borehole when thedownhole motor is activated and the drillstring is rotated. .Iaddend..Iadd.27. A method for controlled directional and straight drilling of aborehole, according to a predetermined well plan, utilizing a bottomholeassembly connected to a drillstring comprising a drill bit connected forindependent rotation from said drillstring to the output shaft of adownhole motor having its uphole end connected to the drillstring,comprising the steps of:1. selecting the placement and diameter of aconcentric stabilizer, the concentric stabilizer being placed close tothe face of the drill bit and having a slightly smaller diameter thanthe borehole, the output shaft of said downhole motor beingconcentrically located within the concentric stabilizer;
 2. selecting apredetermined weight on the drill bit; whereby the assembledconcentrically stabilized bottomhole assembly with the weight on thedrill bit exhibits a certain build angle or drop angle characteristic;3. selecting bend angle oriented in substantially a single directionfrom the centerline of the borehole;
 4. placing the bend angle above thedrill bit on the uphole end of said concentric stabilizer;whereby saidconcentric stabilizer and said bend angle exhibit a certain build anglecharacteristic, the concentrically stabilized bottomhole assembly withthe weight on the drill bit and the bend angle combining to form aninteracting bottomhole assembly having a certain build angle or dropangle characteristic uniquely suited to the well plan; and
 5. steeringsaid unique bottomhole assembly by: (a) turning the drillstring to pointthe drill bit in the desired direction the borehole should follow andactivating the downhole motor while keeping the drillstring stationarywhen it is desired to drill along a curved path, as determined by thebend angle and concentric stabilizer combination; (b) rotating thedrillstring while activating the downhole motor to turn the drill bitwhen it is desired to drill along a straight path. .Iaddend. .Iadd.28. Amethod for controlled directional and straight drilling of a borehole,according to a predetermined well plan, utilizing a bottomhole assemblyconnected to a drillstring comprising a drill bit connected forindependent rotation from said drillstring to the output shaft of adownhole motor having its uphole end connected to the drillstring,comprising the steps of:1. selecting the placement and diameter of aconcentric stabilizer, the concentric stabilizer being placed close tothe face of the drill bit and having a slightly smaller diameter thanthe borehole, the output shaft of said downhole motor beingconcentrically located within the concentric stabilizer;
 2. selecting apredetermined weight on the drill bit; whereby the assembledconcentrically stabilized bottomhole assembly with the weight on thedrill bit exhibits a certain build angle or drop angle characteristic;3. selecting bend angle oriented in substantially a single directionfrom the centerline of the borehole;
 4. placing the bend angle above thedrill bit on the uphole end of said concentric stabilizer;whereby saidconcentric stabilizer and said bend angle exhibit a certain build anglecharacteristic, the concentrically stabilized bottomhole assembly withthe weight on the drill bit and the bend angle combining to form aninteracting bottomhole assembly having a certain build angle or dropangle characteristic uniquely suited to the well plan; and
 5. steeringsaid unique bottomhole assembly by activating the downhole motor andselectively rotating the drillstring. .Iaddend. .Iadd.29. An improvedsystem for controlled directional and straight drilling of a boreholewherein the entire drillstring is rotatable from the surface, includinga bottomhole assembly comprised of a drill bit connected to the outputshaft of a downhole motor for independent rotation from the drillstring,said downhole motor connected at its uphole end to the drillstring, theimprovement in the bottomhole assembly comprising:a first concentricstabilizer, having a preselected diameter slightly smaller than thediameter of the borehole, mounted at a preselected distance from thedrill bit, the output shaft of said downhole motor being concentricallylocated within the first concentric stabilizer; a bent housing connectedabove said first concentric stabilizer and having a bend from itsgeometric center at a predetermined bend angle α, said bend angle αbeing oriented in substantially a single direction from the boreholecenterline, thereby offsetting the centerline of said first concentricstabilizer and the centerline of the drill bit face from the boreholecenterline by said predetermined bend angle α; and a second concentricstabilizer, having a preselected diameter slightly smaller than thediameter of the borehole, mounted at a preselected distance above saidfirst concentric stabilizer;wherein the predetermined bend angle α ofthe bend in the bent housing, and the diameter and placement of theconcentric stabilizers are determined by the desired path of theborehole to be drilled, said system drilling a curved borehole when onlythe downhole motor is activated, and drilling straight borehole when thedownhole motor is activated and the drillstring is rotated. .Iaddend..Iadd.30. A method for controlled directional and straight drilling of aborehole, according to a predetermined well plan, utilizing a bottomholeassembly connected to a drillstring comprising a drill bit connected forindependent rotation from said drillstring to the output shaft of adownhole motor having its uphole end connected to the drillstring,comprising the steps of:1. selecting the placement and diameter of twoconcentric stabilizers as follows, the first concentric stabilizer beingplaced close to the face of the drill bit and having a slightly smallerdiameter than the borehole, the output shaft of said downhole motorbeing concentrically located within the first concentric stabilizer, andthe second concentric stabilizer being placed a predetermined distanceabove the first concentric stabilizer and having a slightly smallerdiameter than the borehole;
 2. selecting a predetermined weight on thedrill bit;whereby the assembled concentrically stabilized bottomholeassembly with the weight on the drill bit exhibits a certain build angleor drop angle characteristic;
 3. selecting bend angle oriented insubstantially a single direction from the centerline of the borehole; 4.placing the bend angle above the drill bit on the uphole end of saidfirst concentric stabilizer;whereby said first concentric stabilizer andsaid bend angle exhibit a certain build angle characteristic, theconcentrically stabilized bottomhole assembly with the weight on thedrill bit and the bend angle combining to form an interacting bottomholeassembly having a certain build angle or drop angle characteristicuniquely suited to the well plan; and
 5. steering said unique bottomholeassembly by: (a) turning the drillstring to point the drill bit in thedesired direction the borehole should follow and activating the downholemotor while keeping the drillstring stationary when it is desired todrill along a curved path, as determined by the bend angle and firstconcentric stabilizer combination; (b) rotating the drillstring whileactivating the downhole motor to turn the drill bit when it is desiredto drill along a straight path. .Iaddend. .Iadd.31. A method forcontrolled directional and straight drilling of a borehole, according toa predetermined well plan, utilizing a bottomhole assembly connected toa drillstring comprising a drill bit connected for independent rotationfrom said drillstring to the output shaft of a downhole motor having itsuphole end connected to the drillstring, comprising the steps of: 1.selecting the placement and diameter of two concentric stabilizers asfollows, the first concentric stabilizer being placed close to the faceof the drill bit and having a slightly smaller diameter than theborehole, the output shaft of said downhole motor being concentricallylocated within the first concentric stabilizer, and the secondconcentric stabilizer being placed a predetermined distance above thefirst concentric stabilizer and having a slightly smaller diameter thanthe borehole;
 2. selecting a predetermined weight on the drillbit;whereby the assembled concentrically stabilized bottomhole assemblywith the weight on the drill bit exhibits a certain build angle or dropangle characteristic;
 3. selecting bend angle oriented in substantiallya single direction from the centerline of the borehole;4. placing thebend angle above the drill bit on the uphole end of said firstconcentric stabilizer;whereby said first concentric stabilizer and saidbend angle exhibit a certain build angle characteristic, theconcentrically stabilized bottomhole assembly with the weight on thedrill bit and the bend angle combining to form an interacting bottomholeassembly having a certain build angle or drop angle characteristicuniquely suited to the well plan; and
 5. steering said unique bottomholeassembly by activating the downhole motor and selectively rotating thedrillstring. .Iaddend. .Iadd.32. A component for a steerable bottomholeassembly for controlled drilling of a borehole, said componentcomprising:a downhole motor having a centerline, an uphole end and adownhole end and a motor housing substantially surrounding said downholemotor; said uphole end of said downhole motor including means forconnecting said downhole motor to a drillstring; said downhole end ofsaid downhole motor including means for connecting said downhole motorto a drill bit; said means for connecting said downhole motor to a drillbit including a bent housing and a bearing assembly; said bearingassembly including a bearing housing, and an output shaft with saidoutput shaft being concentrically located within said bearing housing; afirst concentric stabilizer, having a diameter slightly smaller than thediameter of the borehole and a centerline; said first concentricstabilizer being mounted at a distance below the bent housing; saidoutput shaft of the bearing assembly being concentrically located withinthe first concentric stabilizer; said bent housing having a single bendin one direction located above the first concentric stabilizer andlocate below the downhole motor at a bend angle α, thereby offsettingthe centerline of said first concentric stabilizer from the centerlineof said downhole motor by said bend angle α; a second concentricstabilizer, having a diameter slightly smaller than the diameter of theborehole and a centerline; said second concentric stabilizer beingmounted above said first concentric stabilizer and said bent housing;said centerline of said second concentric stabilizer being offset fromthe centerline of the first concentric stabilizer by said bend angle α;said component for a steerable bottomhole assembly being adapted to aidin steering said bottomhole assembly when activating the downhole motorand selectively rotating the drillstring. .Iaddend. .Iadd.33. Acomponent for a steerable bottomhole assembly for controlled drilling ofa borehole, said component comprising: a downhole motor having acenterline, an uphole end and a downhole end and a motor housingsubstantially surrounding said downhole motor; said uphole end of saiddownhole motor including means for connecting said downhole motor to adrillstring; said downhole end of said downhole motor including meansfor connecting said downhole motor to a drill bit; said means forconnecting said downhole motor to a drill bit including a bent housingand a bearing assembly; said bearing assembly including a bearinghousing, and an output shaft with said output shaft being concentricallylocated within said bearing housing; a first concentric stabilizer,having a diameter slightly smaller than the diameter of the borehole anda centerline; said first concentric stabilizer being mounted at adistance below the bent housing; said output shaft of the bearingassembly being concentrically located within the first concentricstabilizer; said bent housing having a single bend in one directionlocated above the first concentric stabilizer and located below thedownhole motor at a bend angle α, thereby offsetting the centerline ofsaid first concentric stabilizer from the centerline of said downholemotor by said bend angle α; said component for a steerable bottomholeassembly being adapted to aid in steering said bottomhole assembly whenactivating the downhole motor and selectively rotating the drillstring..Iaddend. .Iadd.34. The component for a steerable bottomhole assemblyfor controlled drilling of a borehole of claims 32 or 33 wherein saidbent housing has a bend angle α of one degree or less. .Iaddend..Iadd.35. A component for a steerable bottomhole assembly for controlleddrilling of a borehole, said component comprising:a downhole motorhaving a centerline, an uphole end, and a downhole end, and a motorhousing substantially surrounding said downhole motor; said uphole endof said downhole motor including means for connecting said downholemotor to a drillstring; said downhole end of said downhole motorincluding means for connecting said downhole motor to a drill bit; saidmeans for connecting said downhole motor to a drill bit including a benthousing and a bearing assembly; said bearing assembly including abearing housing, and an output shaft with said output shaft beingconcentrically located within said bearing housing; means forstabilizing said component for a steerable bottomhole assembly; saidmeans for stabilizing said component for a steerable bottomhole assemblyconsisting of a first concentric stabilizer having a diameter slightlysmaller than the diameter of the borehole and a centerline, said firstconcentric stabilizer being mounted at a distance below the benthousing, said output shaft of said bearing assembly being concentricallylocated within the first concentric stabilizer; said bent housing havinga single bend in one direction located above the first concentricstabilizer and located below the downhole motor at a bend angle α,thereby offsetting the centerline of said first concentric stabilizerfrom the centerline of said downhole motor by said bend angle α; saidcomponent for a steerable bottomhole assembly being adapted to aid insteering said bottomhole assembly when activating the downhole motor andselectively rotating the drillstring. .Iaddend. .Iadd.36. A componentfor a steerable bottomhole assembly for controlled drilling of aborehole, said component comprising:a downhole motor having acenterline, an uphole end, and a downhole end, and a motor housingsubstantially surrounding said downhole motor; said uphole end of saiddownhole motor including means for connecting said downhole motor to adrillstring; said downhole end of said downhole motor including meansfor connecting said downhole motor to a drill bit; said means forconnecting said downhole motor to a drill bit including a bent housingand a bearing assembly; said bearing assembly including a bearinghousing, and an output shaft with said output shaft being concentricallylocated within said bearing housing; means for stabilizing saidcomponent for a steerable bottomhole assembly; said means forstabilizing said component for a steerable bottomhole assemblyconsisting of a first concentric stabilizer having a diameter slightlysmaller than the diameter of the borehole and a centerline, said firstconcentric stabilizer being mounted at a distance below the benthousing, said output shaft of said bearing assembly being concentricallylocated within the first concentric stabilizer, and a second concentricstabilizer having a diameter slightly smaller than the diameter of theborehole and a centerline, said second concentric stabilizer being mountabove said first concentric stabilizer and said bent housing; said benthousing having a single bend in one direction located above the firstconcentric stabilizer and located below the downhole motor at a bendangle α, thereby offsetting the centerline of said first concentricstabilizer from the centerline of said downhole motor by said bend angleα; said centerline of said second concentric stabilizer being offsetfrom the centerline of said first concentric stabilizer by said bendangle α; said component for a steerable bottomhole assembly beingadapted to aid in steering said bottomhole assembly when activating thedownhole motor and selectively rotating the drillstring. .Iaddend.